Recurring Complete freeze on new PC build after 30-90 min of use

Hello,
Its my first time consulting a forum such as this to troubleshoot a problem, so feel free to notify me if I'm not using correct protocol.

I've consulted many, many threads on this website and others concerning my problem, but it seems like a "freezing computer" problem is very subjective to individual machines, so here goes.

My brother, who has built many PC's for himself and for me over the years, just finished a new one for me a week ago. After a few days it has started completely freezing (no mouse, keyboard, or C-A-D). This usually happens anywhere between 30 - 120 minutes of use. The screen abruptly freezes up, and any audio playing immediately turns into a horrible screeching sound.

This has occurred while playing a very demanding computer game, but also while watching video using VLC media player, while viewing internet pages, and also several times while transferring video files form my USB connected external hard drive to the new internal hard drive. I have to manually turn off the computer, and when I turn it back on again, its like nothing even happened. No error messages, no memory dump or blue screen, no "your system has recovered from a fatal error" or whatever, and no mention of the crash in the events log or device manager.

I'm running Windows XP SP3, and the following components were all purchased new either on Newegg.com or at BestBuy. I have updated all video drivers off nvidia.com for the graphics card.

AMD Phenom X3 720 2.8GHz Triple-core

MSI 770-C45 Motherboard

Crucial 240-Pin DDR3 SDRAM (2x 2GB)

BFG GeForce GTX 260 896MB PCI Express graphics card

Corsair CMPSU 850W Crossfire Ready Active PFC Power Supply

Western Digital 1Terabyte Hard Drive (SATA)

I have a 100GB Seagate External Hard Drive Connected via USB, as well as a Logitech wireless mouse and keyboard connected via USB.

Too little or too much thermal paste is the first suspect. Then one bad memory module is the usual suspect.
Then unplug all the USB stuff.
Repartition, reformat, and reboot, keeping notes, then check your BIOS, even going back to the previous BIOS.
You know all this stuff, but it is more difficult when something fails to work right and frustration sets in. I would leave it alone for hours or for overnight... Doing the install over from scratch, keeping notes, is also very helpful. Return to your roots... don't work on it while stressed...
Do not ignore the obvious. Cable that is loose, or memory not fully seated. Remove and reseat every plug and socket.

Great, at least a place to start. My brother hadn't thought of the thermal paste possibility, so I'll get some and check on that as well as checking my RAM sticks individually. About the BIOS, nothing has been changed, its a new board and factory settings seemed appropriate? I'll check it out and see whats up.
Thanks

If you are using the same xp disc and the same serial, then when you go on your computer they try to send signals to each other to shut down or freeze because you guys have the same ip address. my friend had this problem, i had to let him borrow a copy of vista from me

Wow, I didn't even think of that. We actually are using the same copy of XP, with the same serial, and we live together and share an internet connection using our Router in our apartment. Honestly, we've been caught up in various possible hardware or incompatibility issues, and haven't even thought of OS problems. The only thing is that my brother's computer runs fine, it hasn't been freezing at all and it sounds like his would be having the same problem if that were the issue. Even still, I think I'll disconnect it from the internet and see if I can't reproduce the freeze while the computer can't connect to the internet.

No OK maybe im explaining it wrong but trust me my friend had this problem and with his brothers computer shutdown his computer dident freeze anymore. So maybe it wasn't the IP address, ill have to ask him but it had to do with something being the same. Oh and when your computer overheats a blue screen will come up and when you start your computer up again it will bring up the windows error message. Trust me my computer used to overheat all the time

SinOfDeath, I think I know what you originally meant. If you have two PCs networked, both running retail copies of Windows XP Pro using the same product key, then yes, you can run into issues. But freezing and shutting down wouldn't be problems, rather, certain features simply wouldn't work.

When your computer overheats, there's no blue screen. It either freezes or the BIOS cuts the power. Windows does not monitor heating, only BIOS would do that and not all BIOSes do. So Windows does not issue a blue screen or error code.